Near-UV (300-375 nm) irradiation of oxygenated, aqueous trptophan solutions gives rise to the mutagenic, cell-lethal agent, hydrogen peroxide, as well as N-formylkynurenine (NFK), an effective, near-UV photosensitizer. These two photoproducts then enhance the photodegradation of biological organisms, including bacteria and bacteriophage. We have found ortho-acetylformanilide (oAF, a close NFK analog) to be an effective, near-UV photosensitizer for nucleoside photooxidation and hydrogen peroxide formation. The proposed work will involve identification of the near-UV, oAF-sensitized oxidation products formed from guanosine, adenosine, thymidine, uridine, and cytidine. The mechanism(s) by which these photosensitized oxidations take place will be investigated. In particular, experiments will be carried out to examine the possible involvement of the superoxide anion and excited state (singlet) oxygen. Near-UV and hydrogen perixide effect synergistic killing of both bacteria (Escherichia coli) and bacteriophage (T7). The proposed work will investigate the chemical basis for the molecular changes which are caused by these agents acting in concert, including the possible involvement of the reaction of hydrogen peroxide with superoxide anion to form the highly oxidizing hydroxyl radical. The genarality of these processes will be investigated, by further examining what other types of near-UV, biological chromophores can act as photosensitizers, what types of bio-molecules are subject to near-UV photosensitized oxidation and hydrogen peroxide generation, and what types of protection against this photooxidative destruction organisms may possess.